The invention relates to a method and system for Enhancing Oil Recovery (EOR) by injecting treated water into an oil bearing formation.
Only a portion of oil present in an oil-bearing formation is recoverable as a result of the natural pressure of the formation. The oil recovered from this “primary” recovery typically ranges from 5% to 35% of the oil in the formation. Enhanced oil recovery methods have been developed to increase the amount of oil that may be recovered from an oil-bearing formation above and beyond that recovered in primary recovery.
Water-flooding, in which water is injected through an injection well into an oil-bearing formation to mobilize and drive oil through the formation for production from a production well, is a widely used method of secondary recovery used to increase the amount of oil recovered from a formation beyond primary recovery. Recently, water-flooding utilizing water having low salinity has been utilized to increase the amount of oil recovered from a formation relative to the amount of oil recovered in a conventional higher salinity water-flood. Low salinity water may be used in place of higher salinity water conventionally used in a water-flood in a secondary recovery, or low salinity water may be used after a conventional higher salinity water-flood to incrementally increase oil recovery over that of the initial water-flood in a tertiary recovery process.
Injection of low salinity water into a formation may reduce the ionic bonding of oil to the formation within pores in the formation by double layer expansion, leading to a reduction in the rock's adsorption capacity for hydrocarbons. This increases the mobility of the oil in the formation by making the surface of the pores of the formation more water-wet and less oil-wet, permitting the mobile oil to be removed from the pores in which it resides and to be driven to a production well for production from the formation.
Low salinity water utilized in low salinity water-flooding typically has a total dissolved solids (“TDS”) content ranging from 200 parts per million (“ppm”) to 5000 ppm, and preferably has a TDS content ranging from 1000 ppm to 5000 ppm to provide adequate salinity in the water to prevent formation damage.
Frequently, the low salinity water provided for enhanced oil recovery is produced by desalinating a source water having significantly higher salinity. Seawater is a common source water treated to provide the low salinity water, particularly for offshore oil recovery. Seawater typically has a TDS content between 30000 ppm and 50000 ppm. Brackish water, high salinity formation water produced from a formation, and high salinity aquifer water may also be utilized as source water that may be desalinated to provide the low salinity source water. Such water sources may have a TDS content ranging from 10000 ppm to 250000 ppm.
Commonly applied technologies for desalination of water include distillation processes, such as Multi Stage Flash, Multi Effect Distillation, Mechanical Vapour Compression and/or Thermal Vapour Compression, and membrane processes, such as Reverse Osmosis (RO), Nano Filtration (NF) and/or Electrodialyses. International patent application WO2011/135048 of Voltea B.V. and the website www.voltea.com disclose a method and apparatus for removal of ions from, for example, wastewater by Capacitive De-Ionisation (CDI). More information on CDI can be found in the scientific paper Environmental Science and Technology, vol. 36/13, page 3017, 2002 and in the article “Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete?” by M. A. Anderson et al. published the Journal Electrochimica Acta 55(2010)3845-3856 and at website www.elsevier.com/locate/electacta.
The latter article by M. A. Anderson et al. shows in FIG. 8 the amount of electrical work required to desalinate water with different salinities and concludes that under the selected conditions and at concentrations below 5000 mg/L, CDI could be a competitive technology even if moderate efficiencies, from 60-70%, are attained.
The most commonly used method for desalination of water used for EOR generally comprises a Micro Filter (MF) or Ultra Filter (UF) assembly for filtering solids from the water and a Reverse Osmosis (RO) assembly or a combination of a nanofiltration assembly and a RO assembly for subsequent water desalination. Several studies on offshore desalination of seawater have concluded that Seawater Reverse Osmosis (SWRO) with either conventional or membrane pre-treatment is by far the most viable desalination method available for offshore application due to suitable weight, cost, footprint, and designed output capacities.
A proper treatment of EOR low salinity injection water is critical to prevent salinity related formation damage. If the clays that are present in the formation are incompatible with the injection water, de-flocculation of the clays could occur. When the clays de-flocculate in the formation, the clay particles may disperse and migrate into the pore throats, resulting in formation damage. In general the injection water/completion fluid must have an adequate salinity (measured in total and/or divalent cation concentration) to prevent de-flocculation of formation clays when the system is in equilibrium. Additionally there must be enough divalent cations (i.e. Ca++, Mg++) present in the displacing fluid (e.g. injected seawater) to prevent de-flocculation of the formation clay during the transition from one water composition to another.
A drawback of both distillation technologies and SWRO's, is that the treated source water has a too high purity requiring blending with seawater or a high salinity membrane retentate stream to adjust the TDS level to the desired levels. Distillation and RO membrane desalination technologies typically reduce the TDS content of the treated source water to less than 500 ppm, often less than 200 ppm. To avoid formation damage, a low salinity water having a TDS of from 1000 ppm to 5000 ppm is desirable, therefore, ions are typically added back to water produced by distillation or RO membrane desalination technologies for use in an EOR application, for example by blending with seawater or with a high salinity membrane retentate stream. Further drawbacks of RO are that RO membranes are sensitive to fouling and RO is energy-intensive.
There is a need to provide an improved and efficient seawater treatment method and system for EOR, which provide treated water with purity, salinity and TDS level suitable for EOR and which therefore do not require subsequent re-blending with raw seawater to re-adjust the TDS level to a desired level, and which is less sensitive to fouling and less energy-intensive than RO.